Electrochemical fuel cell electric power generation systems convert fuel fluid streams, such as natural gas or propane, and oxidant fluid streams, such as oxygen or air, into respective intermediate products, such as a hydrogen-rich fuel stream and a humidified oxidant stream, which a fuel cell ultimately converts into electric power, heat, and reaction products, such as water and carbon dioxide. Fuel cell power plants are of particular interest to utilities because they can provide distributed or remote sources of electricity, thus overcoming some of the difficulties associated with conventional nuclear, coal or hydrocarbon fuel power plants, such as access to high voltage transmission lines, distribution to urban power stations, and the substantial financial commitments typically associated with installation of conventional power plants. In addition, fuel cell power generation systems are capable of operating at greater than 40% electrical efficiency, which is more efficient than combustion-based electric power plants. Fuel cell power generation systems are thus able to use readily available fuels to provide electrical power close to the point of use, quietly, with minimal emissions, and with high overall efficiency.
A hydrocarbon fueled solid polymer fuel cell electric power generation system is the subject of commonly-owned U.S. Pat. No. 5,360,679 issued Nov. 12, 1994 ("the '679 patent") which is hereby incorporated by reference in its entirety. The '679 patent describes a fuel cell generation system that comprises:
(1) an electric power generation subsystem for producing electricity, heat, and water from a hydrogen-containing fuel stream and an oxidant stream; PA1 (2) a fuel processing subsystem for producing a hydrogen-rich fuel for the electric power generation subsystem; PA1 (3) an oxidant subsystem for delivering pressurized oxidant to the electric power generation subsystem; PA1 (4) a water recovery subsystem for recovering the water produced in the electric power generation subsystem and optionally for cooling the electric power generation subsystem; PA1 (5) a power conversion subsystem for converting the electricity produced into utility grade electricity; and PA1 (6) a control subsystem for monitoring and controlling the supply of fuel and oxidant streams to the electric power generation subsystem. PA1 (A) the electric power generation subsystem for cooling fuel cell stack 100; PA1 (B) the fuel processing subsystem for use in the hydrocarbon reforming process and for humidifying the fuel stream fed to fuel cell stack 100; and PA1 (C) the oxidant subsystem for humidifying the oxidant stream fed to fuel cell stack 100. PA1 (A) An electric power generation subsystem comprising at least one fuel cell which comprises a cathode, an anode, and an ion exchange membrane disposed therebetween. The anode has a catalyst associated therewith for producing electrons and protons from a hydrogen-rich fuel stream. The cathode has a catalyst associated therewith for promoting the reaction of oxygen with the protons and electrons to form water and heat. Preferably the electric power generation subsystem comprises a plurality of fuel cells arranged in at least one fuel cell stack. PA1 (B) A fuel processing subsystem for generating the hydrogen-rich fuel stream. The fuel processing subsystem comprising: PA1 (C) An oxidant subsystem for pressurizing an inlet oxidant stream and directing a pressurized oxidant stream to the cathode; PA1 (D) A water circulation subsystem for circulating and recovering water within the fuel cell electric power generation system for humidification of the desulfurized fuel stream and the pressurized oxidant stream and for cooling. PA1 (E) A temperature control subsystem for circulating a temperature control fluid for controlling the temperature within the fuel cell electric power generation system. PA1 (1) a water reservoir for collecting recycled water and receiving make-up water; PA1 (2) a feedwater pump fed for pumping a first portion of circulation water from the water reservoir to a fuel stream humidifier and a second portion of the circulation water to an oxidant stream humidifier; PA1 (3) a first water recovery apparatus for recovering water from the oxidant stream humidifier; PA1 (4) a second water recovery apparatus for recovering water from at least one of the cathode exhaust stream and an anode exhaust stream; and the temperature control subsystem comprises: PA1 (1) a pump for circulating a temperature control fluid through the temperature control subsystem which comprises fluid passages within the electric power generation subsystem; PA1 (2) an indirect heat exchanger for exchanging heat between the temperature control fluid and the second portion of the circulation water; and PA1 (3) a temperature control subsystem heat exchanger for dissipating excess heat from the temperature control subsystem. PA1 (1) a furnace vessel; PA1 (2) a furnace burner comprising a burner head disposed within the furnace vessel for producing a hot burner gas which is circulated within the furnace vessel for providing heat within the furnace vessel; PA1 (3) a reformer disposed within the furnace vessel for endothermically catalytically converting a fuel stream into a reformate fuel stream, which comprises, in addition to hydrogen, carbon monoxide, carbon dioxide, and water vapor, the reformer comprising: PA1 (4) a fluid passage for directing the reformate fuel stream to a reformer outlet through which the reformate stream exits the reformer vessel; PA1 (5) an indirect heat exchanger assembly comprising a pipe with a heat exchanging portion disposed within the furnace vessel for heating a fluid directed through the interior of the pipe, wherein the fluid is directed to the heat exchanging portion from outside the furnace vessel and exits the heat exchanging portion and the furnace vessel through an outlet fluid conduit; and PA1 (6) burner gas fluid passages within the furnace for fluidly isolating the hot burner gas from the fuel stream, the reformate fuel stream, and the fluid flowing through the heat exchanger assembly, wherein the burner gas fluid passages direct the hot burner gas into thermal contact with exterior surfaces of the reformer vessel and the indirect heat exchanger assembly for transferring heat from the hot burner gas to the catalyst bed and the fuel stream within the reformer vessel, and to the fluid flowing through the indirect heat exchanger assembly. PA1 (a) operating an electric power generation subsystem comprising at least one solid polymer fuel cell to electrochemically convert a hydrogen-rich fuel stream and an oxidant stream into reaction products and an electric current; PA1 (b) processing a fuel stream to generate the hydrogen-rich fuel stream, and supplying the hydrogen-rich fuel stream to the electric power generation subsystem, wherein processing the fuel stream comprises: PA1 (c) catalytically reforming the fuel stream to produce a reformate fuel stream using a catalyst and heat from a furnace to promote an endothermic reforming reaction which generates the reformate fuel stream which comprises, in addition to hydrogen, carbon monoxide, carbon dioxide, and water vapor; and PA1 (d) directing the reformate fuel stream through a shift reactor to reduce the amount of carbon monoxide in the reformate fuel stream by reacting carbon monoxide with water in the reformate fuel stream to produce carbon dioxide and hydrogen; PA1 (e) supplying a pressurized oxidant to the cathode using an oxidant compressor to pressurize an inlet oxidant stream; and PA1 (f) directing a cathode exhaust stream from the cathode to the shift reactor for indirectly exchanging heat between the fuel stream and the cathode exhaust stream by passing the cathode exhaust stream through at least one thermally conductive fluid passage within the shift reactor.
The subsystems of the '679 patent are described with reference to FIG. 1, which is a schematic flow diagram of a preferred embodiment of a fuel cell power generation system disclosed in the '679 patent. The electric power generation subsystem comprises fuel cell stack 100. Fuel cell stack 100 preferably comprises a plurality of solid polymer fuel cell assemblies. Each fuel cell assembly comprises a membrane electrode assembly interposed between two separator plates.
The membrane electrode assembly typically employs an ion exchange membrane interposed between two porous, electrically conductive electrodes and a catalyst disposed at the interface between the membrane and the respective electrodes. The separator plates may comprise fluid channels for providing a flow field pattern for directing reactants to the membrane electrode assembly.
In the system illustrated in FIG. 1, the fuel processing subsystem comprises compressor 102, pre-oxidizer cooler 104, pre-oxidizer 106, hydrodesulfurizer 108, hydrogenator 110, evaporator 112, regenerator heat exchanger 114, furnace 116 (comprising a reformer), shift reactor precooler 118, shift reactor first stage 120, intercooler 122, shift reactor second stage 124, hydrogen recycle compressor 126, selective oxidizer pre-cooler 128, selective oxidizer 130, fuel filter 132, anode pre-cooler 134, and water separator 136.
The raw inlet fuel stream is directed to the fuel processing subsystem via compressor 102. Most of the raw inlet fuel stream is directed to downstream fuel processing components. A small portion of the raw inlet fuel stream is directed to auxiliary burner 138.
The raw inlet fuel stream is first directed through preoxidizer cooler 104 and preoxidizer 106. In preoxidizer 106, oxygen from peak shave gas is consumed. Peak shave gas is a mixture of air and propane that is occasionally added to natural gas during peak demand periods. Preoxidizer 106 is not required if the raw inlet fuel stream does not comprise any oxygen, for example, as in the case where peak shave gas is not employed and the raw inlet fuel stream is propane or natural gas.
Next, sulfur is removed from the inlet fuel stream. A desulfurizer such as hydrodesulfurizer 108 may be employed to accomplish this step. The inlet fuel stream that passes through hydrodesulfurizer 108 contacts a catalyst that causes the sulfur to react with hydrogen to form hydrogen sulfide. Hydrogen needed for this reaction is provided by hydrogen recycle compressor 126 which directs a portion of the processed (reformate) hydrogen-rich fuel stream back into the raw inlet fuel stream upstream of hydrodesulfurizer 108. Inside hydrodesulfurizer 108, after contacting the catalyst, the fuel stream then passes over a bed of zinc oxide and the hydrogen sulfide reacts to form solid zinc sulfide and water.
Upon exiting hydrodesulfurizer 108, the desulfurized fuel stream, which still contains some residual hydrogen, is directed to hydrogenator 110 in which it passes through a bed of hydrogenation catalyst that induces the hydrogen to react with unsaturated hydrocarbons (for example, olefins) to produce saturated hydrocarbons.
The fuel stream exiting hydrogenator 110 is then directed to evaporator 112 where the fuel stream is humidified by mixing it with a fine spray of water. For example, evaporator 112 may be a co-current flow vaporizer having a low pressure drop design. The humidified fuel stream exits evaporator 112 at about 350-360.degree. F. (177-182.degree. C.), so the water entrained therein is substantially vaporized. The heat for evaporator 112 is supplied by the burner exhaust stream, which originates from reformer furnace 116.
The humidified fuel stream exiting evaporator 112 is then directed through regenerator heat exchanger 114. In regenerator heat exchanger 114 heat is exchanged between the hot reformate fuel stream exiting furnace 116 and the humidified fuel stream which is being directed toward the reformer in furnace 116. The temperature of the humidified fuel stream leaving regenerator heat exchanger 114 is approximately 650.degree. F. (343.degree. C.).
The humidified and heated fuel stream is then directed to the reformer that is located within furnace 116. A catalyst is provided inside the reformer to induce the desired endothermic chemical reactions that convert the humidified fuel stream into a reformate fuel stream. Furnace burner 140 provides the heat that is required to maintain the desired endothermic reaction. The reformate fuel stream also comprises carbon dioxide, carbon monoxide, and water vapor. The reformate fuel stream leaves reformer furnace 116 with a temperature of approximately 850.degree. F. (454.degree. C.).
As mentioned above, after exiting reformer furnace 116, the reformate fuel stream is directed to regenerator heat exchanger 114 (where the heat from the reformate fuel stream is used to preheat the humidified fuel stream upstream of the reformer). The reformate fuel stream leaving regenerator heat exchanger 114 has a temperature of approximately 580.degree. F. (304.degree. C.). The reformate fuel stream is further cooled in shift reactor precooler 118 where heat is transferred to an oxidant stream before it is fed to furnace burner 140.
The reformate fuel stream exiting shift reactor precooler 118 is then directed to the first stage 120 of a two-stage shift reactor in which a catalyst (preferably a copper-containing composition) exothermically converts the carbon monoxide in the reformate fuel stream into carbon dioxide and hydrogen.
Feedwater preheater 122 acts as a shift reactor intercooler to bring the temperature of the reformate fuel stream exiting first stage 120 of the two-stage shift reactor to approximately 380.degree. F. (193.degree. C.). The reformate fuel stream exiting feedwater preheater 122 is then directed to second stage 124 of the two-stage shift reactor, in which another catalyst is preferably employed to convert carbon monoxide remaining in the reformate fuel stream into carbon dioxide and hydrogen. A small amount of the reformate fuel stream is taken from downstream of second stage 124 and is directed to hydrogen recycle compressor 126 for delivery into the inlet fuel stream upstream of hydrodesulfurizer 108. The remainder of the reformate fuel stream exiting second stage 124 of the two-stage shift reactor is directed through selective oxidizer precooler 128, which cools the reformate fuel stream to approximately 280.degree. F. (138.degree. C.), and is then directed through selective oxidizer 130.
In selective oxidizer 130, the reformate fuel stream is mixed with oxygen to convert substantially all of the remaining carbon monoxide in the reformate fuel stream into carbon dioxide, thus producing a hydrogen-rich fuel stream. Fuel filter 132 removes entrained catalyst particles from the reformate fuel stream exiting selective oxidizer 130. The hydrogen-rich fuel stream exiting filter 132 is then directed to anode precooler 134 which cools the fuel stream to substantially the same temperature as fuel cell stack 100 by spraying water into the fuel stream from the water recovery subsystem.
Before the hydrogen-rich fuel stream is fed into fuel cell stack 100, excess liquid water is separated from the gaseous hydrogen-rich fuel stream in water separator 136. The water exiting water separator 136 is directed to water tank 142. The hydrogen-rich fuel stream exiting water separator 136 is then introduced into the anodes of fuel cell stack 100.
The oxidant subsystem delivers pressurized oxidant to the electric power generation subsystem. Oxidant enters the oxidant subsystem through a conduit and passes through filter 144 to remove particulates. The filtered oxidant stream enters first stage 146 of a two-stage turbocompressor to produce a pressurized oxidant stream having a pressure of approximately 20 psig (138 kPa). The bearing oil associated with first stage 146 of the staged turbocompressor is cooled by circulating the oil through turbocompressor bearing oil cooler 148. The increased pressure of the oxidant stream exiting first stage 146 causes its temperature to rise to approximately 250.degree. F. (121.degree. C.). The pressurized oxidant stream passes through compressor intercooler 150 which cools the oxidant stream to approximately 140.degree. F. (60.degree. C.) for more efficient compression in second stage 152. The pressure of the oxidant stream exiting second stage 152 is approximately 65 psig (448 kPa), and the temperature is approximately 340.degree. F. (171.degree. C.).
A small amount of the pressurized oxidant stream exiting second stage 152 is diverted to auxiliary burner 138 when needed to provide auxiliary energy to drive the turbine portion of the two-stage turbocompressor. The remainder of the pressurized oxidant stream exiting second stage 152 is directed to cathode precooler 154. The oxidant stream exiting cathode precooler 154 has a temperature of approximately 210.degree. F. (99.degree. C.). A small amount of the pressurized, cooled oxidant stream exiting cathode precooler 154 is directed to selective oxidizer 130 where oxygen is needed to selectively oxidize the residual carbon monoxide. The remainder of the pressurized, cooled, oxidant stream is directed to oxidant humidifier 156. In oxidant humidifier 156, coolant water that has passed through the electric power generation subsystem is employed to humidify the oxidant stream. The cooled, humidified oxidant stream exiting oxidant humidifier 156 is further humidified with steam generated by selective oxidizer precooler 128. Finally, the humidified oxidant stream is passed through water trap 158 to remove any water droplets entrained in the oxidant stream. Water removed by water trap 158 is directed to water tank 142. The pressurized, humidified and heated oxidant stream is then introduced into the cathodes of fuel cell stack 100.
Portions of the water recovery subsystem have already been described as they relate to other subsystems. The water recovery subsystem recovers water by collecting excess water from the streams in other subsystems and returning the recovered water to water tank 142. In particular, water is collected from water separator 136 in the fuel processing subsystem, water trap 158 in the oxidant subsystem, water separator 160 which removes excess water from the cathode exhaust stream, water separator 162 which removes excess water from the anode exhaust stream, and water recovery heat exchanger 164 which condenses and removes water from the furnace burner exhaust stream. Feedwater pump 166 is fed from water tank 142 and first pumps recovered water through filter 168 to provide a purified water stream to the following subsystems:
The cooling loop for fuel cell stack 100 comprises a coolant accumulator 170, which acts as a reservoir for coolant thermal expansion and accepts make-up water from feedwater pump 166. Within the cooling loop, coolant pump 172 circulates the cooling water, first to fuel cell stack 100, and then to other components. In FIG. 1, the cooling water that exits fuel cell stack 100 is directed to oxidant humidifier 156. Excess water not absorbed as water vapor in oxidant humidifier 156 is recovered. A portion of the water recovered from oxidant humidifier 156 is directed to selective oxidizer 130. The water exiting from selective oxidizer 130, is combined with the remainder of the water recovered from oxidant humidifier 156, and is directed to low grade heat exchanger 174 and then to temperature control heat exchanger 176, which cools the water stream to approximately 160-170.degree. F. (71-77.degree. C.). The cooled water stream is then directed to coolant pump 172 for recirculation within the coolant loop.
Water directed from water tank 142 to anode precooler 134 is employed to cool the hydrogen-rich fuel stream. Water from water tank 142 is also directed to selective oxidizer precooler 128. The heat transferred from the hydrogen-rich fuel stream to the water flowing through precooler 128 converts the water into steam, which is introduced into the pressurized oxidant stream downstream of oxidant humidifier 156. Finally, water from water tank 142 is also directed to evaporator 112 via shift reactor intercooler 122 (that is, feedwater preheater), which heats the water stream to approximately 310.degree. F. (154.degree. C.). In evaporator 112 the heated water stream is vaporized and combined with the fuel stream for the desired steam reformation reaction.
The power generation system also comprises means for utilizing the surplus oxygen and fuel in the respective cathode and anode exhaust streams, which flow from fuel cell stack 100. The cathode and anode exhaust streams are ultimately directed to furnace burner 140 where these exhaust streams are combusted to produce heat for the reformation process. Prior to introduction to furnace burner 140, the cathode exhaust stream is preheated by being passed through cathode precooler 154, shift reactor precooler 118, and cathode exhaust stream preheater 178. In this way, the cathode exhaust stream removes heat from the oxidant subsystem, the fuel processing subsystem, and the burner exhaust stream, respectively.
The burner exhaust stream provides heat to the cathode exhaust stream in cathode exhaust stream preheater 178 and to the fuel processing subsystem in evaporator 112. After passing through evaporator 112, the burner exhaust stream still has a temperature of about 650-660.degree. F. (343-349.degree. C.); the burner exhaust stream is then directed to auxiliary burner 138 and then to the turbine portion of the two-stage turbocompressors 152 and 146. At full power, the burner exhaust stream produced by furnace burner 140 may provide all the energy required by the turbines to power turbocompressors 152 and 146. Auxiliary burner 138 is typically required during start-up and during conditions when it is needed to provide supplementary energy to the burner exhaust stream.
The burner exhaust stream exiting the turbine portion of turbocompressor 146 has a temperature of about 390.degree. F. (199.degree. C.). The burner exhaust stream exiting the turbines is then directed to high grade heat exchanger 180 where it generates steam, and then to water recovery heat exchanger 164. Inside recovery heat exchanger 164, water vapor in the burner exhaust stream is condensed and a liquid water stream is recovered and directed to water tank 142. The cooled gaseous burner exhaust stream is then expelled from the system.
FIG. 1 also schematically shows inverter 182, which is part of the power conversion subsystem. The present invention relates to improvements that do not directly involve the power conversion subsystem or the control subsystem, and accordingly, these subsystems are not discussed in any detail herein.
An object of the present invention is to provide an improved fuel cell electric power generation system with improved reliability, operability, performance, and which may be manufactured at a reduced cost, compared to previously known solid polymer fuel cell systems. These objectives are achieved, for example, by simplifying the system, reducing the number of components, integrating functions and improving component technology.